sending mails

Hi, I installed Zimbra as an alternative to Outlook. The installation went well and I rceive my mails as it should, BUT ... I cannot send any mails. I get a non delivered mail message with the text : " Invalid Addresses ". Now the addresses are correct and working (checked it out with Outlook) . What can be wrong ? my outgoing server is set to "smtp.one.com" port 2525, as metionned in the setup of my ISP